This invention relates to imaging devices utilizing single crystal semiconductor wafers and more particularly to a silicon vidicon having an improved anti-reflection region for increasing the quantum efficiency of the device in the wavelength range of 400 to 500 nanometers and a method of making the anti-reflection region.
Imaging devices such as silicon vidicons and silicon intensifier tubes employ sensing elements or targets comprising single crystal semiconductor wafers. The operation of such targets in these devices is well known in the art and is described, for example, in U.S. Pat. No. 4,232,245 issued to Savoye et al., on Nov. 4, 1980 and incorporated by reference herein for the purpose of disclosure. Such devices are frequently used in surveillance cameras for monitoring unattended areas. As described in the aforementioned patent, a passivating layer of borosilicate glass is deposited on the input sensing surface of the target. An anti-reflection layer is then deposited on the passivating layer. U.S. Pat. No. 4,228,446 issued to W.M. Kramer on Oct. 14, 1980 and incorporated by reference herein for the purpose of disclosure, describes a device having an anti-reflection layer for a silicon target wherein the anti-reflection layer in combination with the passivating layer forms an anti-reflection region having an optical thickness substantially equal to an odd multiple of a quarter of the wavelength of light incident upon the device. The anti-reflection region described in U.S. Pat. No. 4,228,446 increases the spectral response of the device over a wavelength range from about 400 to 1000 nanometers and enhances the quantum efficiency in the wavelength range of about 550 to 600 nanometers. However, the quantum efficiency of such a device over the wavelength range of 400 to 500 nanometers decreases rapidly as the wavelength of incident light decreases.
Many illumination sources which are used in conjunction with surveillance cameras have spectral distributions with intensity peaks occuring below the quantum efficiency peak of 550 to 600 nanometers described in U.S. Pat. No. 4,228,446, referenced above. For example, a mercury arc lamp has an intensity peak at about 455 nanometers and a fluorescent daylight lamp has an intensity peak at about 450 nanometers. It is, therefore, desirable to increase the quantum efficiency of the imaging device within the wavelength range of 400 to 500 nanometers without substantially decreasing the quantum efficiency within the range of 550 to 600 nanometers.